docs: indicate stable version in selector on deploy (#654)

Adds an indicator in the form of `$version (stable)` to make it easier to see which is the most recent stable version.

This is done via CI and mike:

1. Set the previous title to just `$version`. We can get this value by `mike list stable` and grepping for valid version tags (otherwise this might have issues with `$version (stable)`).
2. Deploy `$new_version` and alias to stable.
3. Update the `$new_version` to have a new title of `$new_version (stable)`.

.github/workflows/post-release.yml

@@ -47,10 +47,14 @@ jobs:
           echo Name: $(git config --get user.name)
           echo Email: $(git config --get user.email)
 
-      - name: Build and deploy docs with mike
+      - name: Build and deploy docs with mike as the latest stable branch
         run: |
           cd docs
+          OLD_STABLE_VERSION=$(mike list stable | grep -Po '([0-9]+.[0-9]+.[0-9]+)' | head -n1)
+          echo ${OLD_STABLE_VERSION}
+          mike retitle --push stable ${OLD_STABLE_VERSION}
           mike deploy --push --update-aliases ${RELEASE_VERSION} stable
+          mike retitle --push ${RELEASE_VERSION} "${RELEASE_VERSION} (stable)"
 
       # - name: Deploy to CF Pages
       #   run: |